Viscose Rayon

Viscose Rayon Introduction Rayon fiber made from “Viscose Process” is called Viscose Rayon. Viscose Rayon is the oldest commercial man-made fiber. Although Viscose Rayon is a man-made fiber, it is not a purely synthetic fiber. Viscose Rayon is manufactured from wood pulp, a naturally-occurring, cellulose-based raw material. Usually, logs of a special kind of wood, spruce, are used which is also used in paper industry as raw material. On the basis of its raw material and its manufacturing process, Viscose Rayon is called “Semi-Synthetic Fiber”. Inshort, Viscose Rayon is a manufactured regenerated cellulosic fiber composed of regenerated cellulose. Hence, properties of Viscose Rayon are more like natural cellulosic fibers, such as lint or cotton, than those of thermoplastic, petroleum- based synthetic fibers such as Nylon, Polyester etc. Out of all the fibers produced, Viscose Rayon is probably the most perplexing to consumers. It can be found in cotton-like end uses as well as silk-like end uses. Due to its fine silk-like properties, it is also known as “Artificial Silk”. History Viscose Rayon is the very first viable manufactured fiber and artificial silk fiber. In 1664, English naturalist Robert Hooke gave a theory that artificial silk filaments can be spun from a substance similar to that which silkworms utilize to make silk. Until 1885, scientists from all over the world tried to make artificial silk following Robert Hooke’s theory but nobody succeeded. In 1885, Frenchman George Audemars was succeeded to some extent in making artificial silk thread by dipping a needle into viscous solution of mulberry bark pulp and gummy rubber but this process was hardly economically viable because of the requirement of great deal of skill and precision. In 1889, Hilaire de Berniguad produced first commercial synthetic fiber. It was patented in 1884 by him and was known as “Chardonnay Silk” but it also was not economically viable as it was very inflammable. In 1891, the English chemist Charles Frederick Cross and Edward John Bevan along their collaborator Clayton Beadle discovered “Viscose Process”. In 1892, C. F. Cross and E. J. Bevan took out a British Patent no. 8700, “Improvements in Dissolving Cellulose and Allied Compounds” for Viscose that became the basis for the viscose and rayon industries. In 1893, the duo formed the “Viscose Syndicate” to grant licenses for “Viscose Process”. In 1905, Courtaulds Ltd. USA produced the first commercial Viscose Rayon which was very first economically viable “Artificial Silk”. Chemistry of Viscose Process First of all, to prepare viscose, the cellulose [C6H10O5]n is treated with sodium hydroxide [NaOH] aka “Caustic Soda” to form alkali cellulose or soda cellulose which has the approximate formula [C6H9O4-ONa]n. The alkali or soda cellulose is then treated with carbon disulfide [CS2] to form a solution of sodium cellulose xanthate [C6H9O4-OCS2Na]n, which is called viscose. The higher the ratio of cellulose to be combined with sulphur, the lower the solubility of the cellulose xanthate. The sodium cellulose xanthate [C6H9O4-OCS2Na]n is dissolved in additional dilute sodium hydroxide [NaOH] and allowed to depolymerize or ripen to a desired extent. The rate of depolymerization or ripening depends on temperature and is affected by the presence of various inorganic and organic additives, such as metal oxides and hydroxides. Air also affects the ripening process since oxygen causes depolymerization. Rayon fiber is produced from the ripened solutions by treatment with a mineral acid, such as sulphuric acid. In this step, the xanthate groups are hydrolyzed to regenerate cellulose. In regeneration of cellulose, polymer chains are shortened. This is known as “Degradation of Cellulose”. Flow Sheet Diagram of Viscose Process Manufacturing of Viscose Rayon Chemical Reactions [C6H10O5]n + nNaOH [C6H9O4-ONa]n + nH2O Cellulose Caustic Soda Soda Cellulose [C6H9O4-ONa]n + nCS2  [C6H9O4-OCS2Na]n Soda Cellulose Sod. Cellulose Xanthate [C6H9O4-OCS2Na]n + ½ H2SO4 → [C6H10O5]n + CS2 + ½ Na2SO4 Sod. Cellulose Xanthate Viscose Rayon Viscose Process It is the worldwide manufacturing process of “Viscose Rayon”. Raw Materials Wood Pulp Cotton Waste Cotton Linters Viscose Process is as follows: Preparation of Wood Pulp: The first step in the manufacturing of Viscose Rayon is preparation of wood pulp. Spruce trees are cut into timber. Their barks are removed and cut into pieces of about 7/8" x 1/2" x 1/4". These pieces are treated with a solution of calcium bisulphite [CS2] for 8-12 hours and cooked with steam under pressure for about 14 hours. The cellulosic component of the wood is unaffected by this treatment, but the cementing material called lignin, which is present in the wood, is converted into its sulphonated compound which is soluble in water. Sulphonated compound is washed off, hence purifying the remaining cellulose. After this it is treated with sodium hypochlorite solution and finally converted into paper boards or sheets. This is called wood pulp, which is normally purchased by the manufacturers of viscose rayon. Steeping & Pressing: The wood pulp sheets are treated with 17-20% aqueous caustic soda at a temperature 18-25°C. It is called “Mercerizing or Steeping Process”. The high DP cellulose (1000) is converted into soda cellulose. The sheets are allowed to soak until they become dark brown in colour. This takes about 1-4 hours. The excess caustic soda solution is drained off and sheets are pressed out by a hydraulic press to squeeze out excess caustic soda solution. This process is known as “Pressing”. 100 kg of sulphite pulp gives about 310 kg of moist soda cellulose. Recovery of NaOH: Pure caustic soda is recovered from the moist soda cellulose formed in the previous step by allowing diffusion of moist soda cellulose through paper membranes known as “parchment”. Recovery of caustic soda and reuse of caustic soda makes it an economically viable process. To reuse the used caustic soda, it is concentrated and mixed with fresh caustic soda. When reused caustic soda is exhausted and can no longer give required result, it is sold to soap making industries and is used in soap manufacturing process. Shredding: The pressed soda cellulose is shredded mechanically to yield finely divided, fluffy particles called "crumbs". This step provides increased surface area of the soda cellulose, thereby increasing its ability to react in the steps that follow. The wet, soft sheets of soda cellulose are passed through a shredding machine which cuts them into small bits. The sheets are broken into fine crumbs in 2-3 hours. Aging: The soda cellulose is aged under controlled conditions of time and temperature (between 18 and 30°C) in order to depolymerize the cellulose to the desired degree of polymerization to obtain almost ideal solution of cellulose. The soda cellulose is stored in small galvanized drums for about 48 hours at 28°C. This process is called “Ageing”. In this step, the average molecular weight of the original pulp is reduced by a factor of 2-3. Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration. During this process, the Degree of Polymerization of soda cellulose is decreased from 800 to about 350. Xanthation: After ageing, the crumbs of soda cellulose are transferred to rotating, air tight, hexagonal drum called “Churner”. In this step, the aged soda cellulose crumbs are placed in rotating drums and are allowed to react with carbon disulphide under controlled temperature to form sodium cellulose xanthate. Carbon disulphide about 10% of the weight of the crumbs is added to the churner and churned together for 3 hours by rotating the mixers at a slow speed of 2 revolutions per minute [rpm]. Sodium cellulose xanthate is formed during this process and the colors of the product changes from white to reddish orange. Side reactions that occur along with the conversion of soda cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and the resulting viscose solution. Dissolving: Sodium cellulose xanthate is in the form of small balls. This falls into a mixer called dissolver which is provided with a stirrer. A dilute solution of caustic soda is added, and the contents are stirred for 4-5 hours and at the same time, the dissolver is cooled. The sodium cellulose xanthate dissolves to give clear brown thick liquor, similar to honey. This is called 'viscose' and it contains about 6.5% caustic soda and 7.5% cellulose. Ripening: Viscose solution requires ripening to give a solution having best spinning qualities. Two important processes occur during ripening: Redistribution and loss of xanthate groups. Ripening is carried by storing the viscose solution for 4-5 days at 10-18°C. The viscosity of the solution first decreases and then rises to its original value. The ripened solution is filtered carefully and is now ready for spinning to produce viscose rayon filaments. Spinning: In spinning process, ripened viscose solution first goes through a centrifugal pump due to pressure exerted on the solution by compressed air. Then it is delivered to a “Filtering Media”. From there, this solution goes into the “Glass Tubing” which has a spinneret. The viscose solution is forced through a spinneret, having many fine holes with diameters ranging from 0.05-0.1mm. The spinneret is submerged into a solution containing the following chemicals: Water [H2O] = 69% Sodium Sulphate [Na2SO4] = 18% Sulphuric Acid [H2SO4] = 10% Glucose [C6H12O6] = 2% Zinc Sulphate [ZnSO4] = 1% The spinning solution is kept at 40-45°C. Sodium sulphate precipitates the dissolved sodium cellulose xanthate. Sulphuric acid converts xanthate into cellulose, carbon disulphide and sodium sulphate. Glucose gives softness and pliability to the filaments whereas zinc sulphate gives added strength. As soon as a number of filaments emerge from the spinneret, they are taken together to surface of the spinning bath and then guided to two rollers from where they are wound on to a spindle. Structure of Viscose Rayon Different kind of fibers can be identified by their microscopic appearance better, than their physical appearance. When viscose rayon is viewed under microscopic lens; In longitudinal view, a regular Viscose Rayon exhibits uniform diameter and interior parallel lines called striations. In cross section view, viscose fiber shows highly irregular or serrated edges. Cross Sectional Longitudinal Properties of Viscose Rayon Physical Properties Tensile Strength Tensile Strength of the fiber is greater when the fiber is dry as compared to its wet state. Because of this, it stretches and shrinks more than cotton. Tensile strength of Viscose Rayon ranges from 30,000-46,000 lb. /inch2. Tenacity It is 1.5-2.4 gpd in the dry state and 0.7-1.2 gpd in the wet state. Wet strength of fiber is of importance during its manufacture and usage. Modifications in the production have led to the problem of low wet strength being overcome. Elastic Properties Viscose Rayon has very low elasticity. Recovery from deformation of this fiber from applied load is very low as it is a rigid fiber and inelastic. At 1% extension, it has elastic recovery of 67%. At 2% extension, it has elastic recovery of 60%. At 5% extension, it has elastic recovery of 32%. At 10% extension, it has elastic recovery of 23%. Elongation at Break Regular Viscose Rayon has elongation at break of 15% when in dry state and has elongation at break of 25% when in wet state. High tenacity rayon has elongation at break of 17%. Moisture Absorption It absorbs more moisture than cotton. Moisture Content of Cotton is 6% at 70F whereas for Viscose Rayon, it is 13% under the same conditions. Its ability to absorb perspiration and allowance it to evaporate away is exceptional. Specific Gravity Specific gravity ranges from 1.46-1.54. Density The density of Viscose rayon is 1.53 g/cm3. Effect of Heat At 149 C or more, Viscose Rayon loses its strength and begins to decompose at 177-204C. It doesn’t melt or stick at elevated temperatures. Effect of Sunlight Prolonged exposure to sunlight also weakens the fiber due to moisture and ultraviolet light of the sunlight. Effect of Age There is a gradual loss of strength with age if stored carefully. Abrasion Resistance Its abrasion resistance is very low thanks to the inelasticity of rayon. It is easily damaged by scraping. It has poor crease recovery and crease retention. Moisture Regain Moisture regain is 11-13%. Electrical Properties Owing to its high moisture absorption, it doesn’t show insulation properties. Static charges are not also developed in textile working. Chemical Properties Action of Acids The resistance of Viscose Rayon to acids is generally less than that of cotton. Organic acids can be safely used in 1-2% concentration without injury to the fiber. Inorganic acids such as hydrochloric & nitric acids can be used in surprisingly strong concentrations provided the temperatures are not too high and the treatment is brief. At high temperatures and concentrations, all acid will destroy or carbonize it. Inshort, it is damaged by strong acids but it is moderate with weak acids. Action of Bases Viscose rayon has a good resistance to weak alkalis but strong alkalis can harm theses fibers thoroughly. Action of Dry Heat Viscose Rayon, under the influence of heat as well as light, shows rapid loss in strength. Degradation of cellulose is lower in the absence of oxygen. Continued heating in the absence of oxygen leads to deterioration of the cellulose. Action of Solvents Textile solvents can be used on Viscose rayon without any deteriorating effect. For example, Viscose rayon dissolves in cuprammonium hydroxide solution. Effect of Iron Contact with iron in the form of ferrous hydroxide weakens Viscose Rayon. Therefore, staining, marking or touching of rayon to iron or iron surface should be avoided. Biological Properties Microorganisms such as moulds and mildews affect the colour, strength, dyeing properties and luster of Rayon. Clean and dry Viscose Rayon is rarely attacked by moulds and mildew. Moth larvae eat it but can’t digest it and excretes unchanged. End Uses Yarns : Embroidery thread, cord, Novelty yarns. Fabrics : Crepe, Suiting, Lace, Outerwear fabrics, Linings for fur coats. Apparel : Blouses, Dresses, Saris, Jackets, Lingerie, Hats, Slacks, Sport shirts, Suits, Ties, Work clothes Domestic Textiles : Bedspreads, Blankets, Curtains, Draperies, Sheets. Industrial Textile : High tenacity rayon is used as reinforcement to mechanical rubber goods such as tires, conveyer belts, hoses etc. References S. P. Mishra. (2000) A Text Book of Fiber Science and Technology, New Delhi: New Age International (P) Ltd., Publishers. J. Gordon Cook. (1984) Handbook of Textile Fibers, VOL.-II. Manmade Fibers, 5th edition, London: Merrow Publishing Co. Ltd., Menachem Lewin. (ed.) (2006) Handbook of Fiber Chemistry, 3rd Edition, London: CRC Press. Joseph, Marjory L. (1972) Introductory Textile Science, CA, U.S.A: Holt, Rinehart and Winston. Ashish Kumar Dua. (2013) Viscose Rayon Manufacturing Process. [Online] Available at: <http://textilelearner.blogspot.com/2013/06/viscose-rayon-manufacturing-process.html> [Accessed 24th August 2013]. Md. Mazharul Islam Kiron. (2012) Properties of Rayon Fiber. [Online] Available at: <http://textilelearner.blogspot.com/2012/04/properties-of-rayon-fiber-physical-and.html> [Accessed 24th August 2013]. Swicofil. (201?) Viscose Rayon. [Online] Available at: <http://www.swicofil.com/viscose.html> [Accessed 25th August 2013]. Textilefashionstudy. (2012) Viscose Rayon / Physical and Chemical Properties of Viscose Rayon. [Online] Available at: <http://textilefashionstudy.com/viscose-rayon-physical-and-chemical-properties-of-viscose-rayon/> [Accessed 25th August 2013]. Fibersource. (200?) Rayon Fiber (Viscose). [Online] Available at: <http://www.fibersource.com/f-tutor/rayon.htm> [Accessed 25th August 2013]. Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath. (2004) Rayon Fibers. [Online] Available at: <http://www.engr.utk.edu/mse/Textiles/Rayon%20fibers.htm> [Accessed 25th August 2013]. textilelearner.blogspot. (2012) Characteristics of Rayon Fiber. [Online] Available at: <http://textilelearner.blogspot.com/2012/03/rayon-fiber-characteristics-of-rayon.html> [Accessed 25th August 2013]. mytextilenotes.blogspot. (2009) Manufacturing Process of Viscose Rayon. [Blog] 12 May. Available at: <http://mytextilenotes.blogspot.com/2009/05/manufacturing-process-of-viscose-rayon.html> [Accessed 25th August 2013]. 4textile.blogspot. (2012) Chemical Reaction for Viscose Rayon Production. [Online] Available at: <http://4textile.blogspot.com/2012/10/222.html> [Accessed 25th August 2013]. Wikipedia. (n.d.) Viscose. [Online] Available at: <http://en.wikipedia.org/wiki/Viscose> [Accessed 25th August 2013]. Wikipedia. (n.d.) Rayon. [Online] Available at: < http://en.wikipedia.org/wiki/Rayon> [Accessed 25th August 2013]. Wikipedia. (n.d.) Charles Frederick Cross. [Online] Available at: <http://en.wikipedia.org/wiki/ Charles_Frederick_Cross> [Accessed 25th August 2013].